1. Field of the Invention
The present invention relates to the gallium-nitride (GaN) based light-emitting-diode (LED) structure, and in particular to the gallium-nitride (GaN) based light emitting diode which achieves the increased light transmittance and enhanced light illuminance by means of its top transmittance contact layer using ITO (indium-tin-oxide) as its material.
2. The Prior Arts
It is known, the indium-gallium-nitride (InGaN)/gallium-nitride (GaN) multi-quantum well (MQW) LED is usually used in the prior art as the light emitting device. It has been widely utilized in the various functions and applications of static display, such as, in the clocks/watches, display screens, and advertisement panels, etc for displaying digits and/or images. However, its light illuminance and light transmittance efficiency are restricted by the property of its top transparent contact layer, and its light transmittance at present can only reach 62% at most in the visible light spectrum. Therefore, the effectiveness of its application is not very satisfactory.
Now, please refer to FIG. 1 as we explain the structure of this prior art multi-quantum well light emitting diode (MQW LED) and its restrictions. As shown in FIG. 1, its structure comprises a substrate 11, a buffer layer 12, an un-doped n-type gallium-nitride (GaN) layer 13, an n-type gallium-nitride (GaN) layer 14, a multi-quantum well (MQW) layer 15, a gallium-nitride (GaN) cladding layer 16, a p-type gallium-nitride (GaN) layer 17, a Ni/Au transparent contact layer 18, and a Ni/Al transparent contact layer 19.
In the above-mentioned structure, the lowest layer is the substrate 11 and it is made of Sapphire. The buffer layer 12 is formed on the substrate 11 and is made of low temperature growth gallium-nitride (GaN). The un-doped n-type gallium-nitride (GaN) layer 13 is formed on the buffer layer 12. Then, the n-type gallium-nitride (GaN) layer 14 is formed on un-doped n-type gallium-nitride (GaN) layer 13. Afterwards, the multi-quantum well (MQW) layer 15 is formed on the n-type gallium-nitride (GaN) layer 14, and it is made of InGaN/GaN. Formed on top of the multi-quantum well layer is the gallium-nitride (GaN) cladding layer 16. Then, p-type GaN layer 17 is formed on gallium-nitride (GaN) cladding layer 16. Finally, Ni/Au transparent contact layer 18 is formed on p-type GaN layer 17, and the Ni/Al transparent contact layer 19 is formed on the n-type gallium-nitride (GaN) layer 14.
This sort of prior art multi-quantum-well light emitting diode structure is known as the “n-Down Structure”. Namely, in this structure, the InGaN/GaN multi-quantum well (MQW) active layer is grown on the n-type (n-GaN) cladding layer, and then the p-type GaN cladding layer is grown on the multi-quantum well active layer. The purpose of the light emitting diode produced in this manner is to take advantage of the superb crystal quality of the multi-quantum well (MQW) active layer, so as to achieve better current distribution in the underlying n-GaN layer, and therefore the lower turn-on voltage of the light emitting diode.
However, the main purpose of this kind of prior art multi-quantum-well light emitting diode (InGaN/GaN MQW LED) is to make use of the n-type (n-GaN) layer as the contacting layer, and the Ni/Au as the p-type conductive electrode and the transparent contact layer. According to the experiment data of FIG. 2, the maximum transmittance (namely, the transmitted percentage of the incident light) of that Ni/Au transparent contact layer in the visible light spectrum is only 62% at 530 nm. Therefore, the light emitting diode of the prior art is restricted by its intrinsic light transmittance property of this transparent contact layer, and as such its light illuminance can not be raised, and that is its major shortcomings and restrictions.
The purpose of the present invention is to improve the shortcomings and the restrictions of the afore-mentioned conventional light emitting diode, so as to achieve the purpose and function of increasing its light illuminance and light emission efficiency.